In many processes where smoke cannot be eliminated or where black-body conditions are not satisfactory, a porcelain or other refractory tube with a closed end is inserted into the furnace. The pyrometer is sighted into this tube which if fairly uniformly heated over a sufficient area affords an excellent black body. This method has been employed also for obtaining the true temperature of molten metals but suitable refractory tubes for many molten metals have yet to be developed.
Radiation Pyrometry.—An optical pyrometer measures the intensity of a narrow spectral band of radiation emitted by a glowing object. The radiation pyrometer measures the intensity of all wave lengths, the light rays and the heat rays combined. Usually the energy of all wave lengths radiated by the source is focused in some manner upon the hot junction of a small thermocouple. The temperature to which this junction rises is approximately proportional to the rate at which energy falls upon it, which in turn, by the Stefan-Boltzmann law, is proportional to the fourth power of the absolute temperature of the source. The rise in temperature of the hot junction of the couple generates a thermoelectric electromotive force. Hence, the calibration of a radiation pyrometer consists in determining the relation between the electromotive force developed and the temperature of the source sighted upon. This relation follows the law e = a&b where # is the absolute temperature of the source, e is the electromotive force developed by the instrument and a and b are empirical constants determinable by two standardization points. The electromotive force may be measured by a potentiometer or galvanometer, or by any of the methods discussed under thermoelectric pyrometry. The galvanometer should have as h1'^ n ™™«+*•"'»*
FIG. 21.—Thwing radiation pyrometer.
as is consistent with the requirement of robustness. The same type of instrument is used with the radiation pyrometer as with the ordinary thermocouple. In fact the entire discussion on galvanometric methods of measuring electromotive force of thermocouples is equally applicable to radiation pyrometry. The temperature of the cold junction of the couple in the radiation pyrometer is not controlled. The hot and cold junctions are in fairly close proximity and are hence equally affected by changes in room temperature. The cold junction is always shaded from the heat radiated by the source sighted upon.
Figure 21 illustrates the principle of the Thwing radiation pyrometer made by the Thwing Instrument Co. Radiation from the furnace enters the diaphragm A and falls upon the hollow conical mirror K. The hot junction C of a minute thermocouple is located at the apex of the cone and the cold junctions are at D and D'. By multiple reflection along the sides of the conical mirror the radiation is finally concentrated upon the hot junction of the couple. The electromotive force is measured by a galvanometer graduated to read temperature directly. Except for incidental errors which will be considered later, the reading of the instrument is independent of the sighting distance provided the diameter of the source is sufficient to fill the cone of rays defined by the geometrical construction of the receiving tube. The